1. Field of the Disclosure
Embodiments disclosed herein relate generally to the alkylation of paraffinic hydrocarbon feedstocks wherein olefins are reacted with isobutane and isopentane to produce a low Reid vapor pressure alkylate product.
2. Background
Alkylation is the reaction of a paraffin, usually isoparaffins, with an olefin in the presence of a strong acid which produces paraffins, e.g., of higher octane number than the starting materials and which boil in the range of gasolines. In petroleum refining, the reaction is generally the reaction of a C3 to C5 olefin with isobutane.
Hydrofluoric or sulfuric acid catalysts are commonly used in refining alkylations. For sulfuric acid catalyzed alkylation, low temperature or cold acid processes are favored, minimizing side reactions. In a typical process, the alkylation is carried out in a reactor where the hydrocarbon reactants are dispersed into a continuous acid phase.
For example, U.S. Pat. No. 2,762,853 discloses an alkylation process including feeding isoparaffins, such as isobutane or isopentane and C2-C5 monoolefins to an alkylation reactor. The alkylation reaction is catalyzed with sulfuric acid in excess of 88 percent, preferably in excess of 96 percent H2SO4. The alkylation products are then separated into gasoline range components and heavier alkylate products, among other finishing processes. Other alkylation processes are disclosed in, for example, U.S. Pat. Nos. 2,859,260 and 3,013, 092.
U.S. Pat. No. 6,995,296 discloses a process for the alkylation of alkane with olefin or olefin precursor such as an oligomer of tertiary olefin. The alkylation may be performed by contacting an acid catalyst, isoparaffin and olefin in concurrent downflow with a disperser mesh in a reaction zone under conditions of temperature and pressure to react the isoparaffin and the olefin to produce an alkylate product. The olefin precursor may include an oligomer comprising C8 to C16 olefins corresponding to oligomers prepared from C3 to C5 olefins. Instead of the expected reaction between the oligomer and the isoalkane, the oligomer is split into its olefin constituents that react with the isoalkane on a molar basis.
Recent reformulated gasoline specifications require a reduction in both the olefin content and the Reid Vapor Pressure (RVP) of the gasoline. Alkylate is a low vapor pressure, high octane gasoline blending component containing no olefins. Thus, alkylate helps refiners meet the new reduced RVP and reduced olefins content specifications.
As practiced commercially, alkylate product resulting from the reaction of C3 to C5 olefins with isobutane is typically C7 to C9 isoparaffins, along with lesser amounts of lighter and heavier isoparaffins in the C6 to C12 range. Isopentane is also produced during the alkylation reaction. The production of isopentane during alkylation of isobutane may increase isopentane concentration in the final gasoline pool, and, with an RVP of 20.5 psi, negatively affects the desired or targeted RVP and resultant yield.
Alkylation of isopentane is one method to reduce its RVP. However, alkylation of pure isopentane or suppression of isopentane products from amylene alkylation results in low C5+ yields, lower octane value products relative to isobutane alkylates, and low isopentane conversions.
U.S. Pat. No. 5,583,275 discloses an alkylation process in which C3 to C5 olefins are reacted with an isoparaffin mixture of isopentane and isobutane in the presence of a catalyst. As disclosed therein, varying the amount of isopentane present in the isoparaffin mixture can control or eliminate the amount of isopentane produced in the alkylation reactor, and may produce an alkylate having a lower RVP and may result in a lower olefin content for reformulated gasoline.
U.S. Pat. No. 6,429,349 discloses an alkylation process in which low purity isopentane is added to an alkylation reactor to block formation of isopentane, which may result in high incremental isopentane conversion and minimal octane and C5+ yield loss, as well as low acid consumption from C6+ isoparaffins.
Accordingly, there exists a need for alkylation processes that may provide for reduced olefin content in reformulated gasoline and allow for a decrease in RVP.